Memory with bit line current injection

ABSTRACT

Embodiments of a memory are disclosed that may allow for the detection of weak data storage cells or may allow operation of data storage cells under conditions that may represent the effects of transistor ageing. The memory may include data storage cells, a column multiplexer, a sense amplifier, and a current injector. The current injector may be configured to generate multiple current levels and may be operable to controllably select one of the current levels to either source current to or sink current from the input of the sense amplifier.

PRIORITY INFORMATION

This application is a continuation of U.S. patent application Ser. No.13/409,399 filed on Mar. 1, 2012, which is incorporated by referenceherein in its entirety.

BACKGROUND

1. Technical Field

Embodiments described herein are related to the field of memoryimplementation, and more particularly to techniques for data storagecell testing.

2. Description of the Related Art

Memories typically include a number of data storage cells composed ofinterconnected transistors fabricated on a semiconductor substrate. Suchdata storage cells may store a single data bit or multiple data bits andmay be constructed according to a number of different circuit designstyles. For example, the data storage cells may be implemented as asingle transistor coupled to a capacitor to form a dynamic storage cell.Alternatively, cross-coupled inverters may be employed to form a staticstorage cell or a floating gate MOSFET may be used to create anon-volatile storage cell.

During the semiconductor manufacturing process, variations inlithography, transistor dopant levels, etc., may result in differentelectrical characteristics between transistors that are intended to haveidentical characteristics. Additional variation in electricalcharacteristics may occur due to aging effects within transistors as thedevice is repeatedly operated. These differences in electricalcharacteristics between transistors can result in data storage cellsthat output different small signal voltages for the same stored data. Ina memory array, there may be a large variation in the small signaloutput voltages across the data storage cells that comprise the memoryarray.

Data from data storage cells that generate a smaller than average outputsignal due to the previously described variation may not be able to beread correctly, resulting in a misread. Data storage cells that fail toread properly may contribute to lower manufacturing yield andnecessitate additional redundant data storage cells to maintainmanufacturing yield goals.

SUMMARY

Various embodiments of a memory circuit are disclosed. In an embodiment,the memory circuit may include data storage cells, a column multiplexer,a sense amplifier, and a current injector. The current injector maysource current to the input of the sense amplifier. In some embodiments,the current injector may be operable to controllably source current toone of the inputs of sense amplifier configured to amplify adifferentially encoded signal.

The current injector may be configured to generate multiple currentlevels and may be operable to controllably select one of the currentlevels to source to the input of the sense amplifier. In someembodiments, the current injector may be configured to sink current fromthe input of the sense amplifier.

During operation, test data may be stored into a data storage cell. Thestored data may be read from the data storage cell using a senseamplifier and compared to the original test data. The stored data mayalso be read from the data storage cell using the sense amplifier whilesourcing current to the input of the sense amplifier and compared to theoriginal test data. The result of these comparisons may be used todetermine the strength of the data storage cell. In some embodiments,information indicative of the strength of the data storage cell may bestored for later use.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 illustrates an embodiment of a data storage cell.

FIG. 2 illustrates possible waveforms for the discharge of bit lines.

FIG. 3 illustrates an embodiment of a memory sub-array.

FIG. 4 illustrates a possible method of operating the embodimentillustrated in FIG. 3.

FIG. 5 illustrates an embodiment of a current injector.

FIG. 6 illustrates an embodiment of a current injector with multiplecurrent levels.

FIG. 7 illustrates an embodiment of a memory.

FIG. 8 illustrates a possible method of operating the embodimentillustrated in FIG. 7.

FIG. 9 illustrates a possible method for reading a memory and comparingthe stored data to previously loaded test data.

FIG. 10 illustrates an embodiment of a computing system.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the disclosure to theparticular form illustrated, but on the contrary, the disclosure is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present disclosure as defined by the appendedclaims. The headings used herein are for organizational purposes onlyand are not meant to be used to limit the scope of the description. Asused throughout this application, the word “may” is used in a permissivesense (i.e., meaning having the potential to), rather than the mandatorysense (i.e., meaning must). Similarly, the words “include,” “including,”and “includes” mean including, but not limited to.

Various units, circuits, or other components may be described as“configured to” perform a task or tasks. In such contexts, “configuredto” is a broad recitation of structure generally meaning “havingcircuitry that” performs the task or tasks during operation. As such,the unit/circuit/component can be configured to perform the task evenwhen the unit/circuit/component is not currently on. In general, thecircuitry that forms the structure corresponding to “configured to” mayinclude hardware circuits. Similarly, various units/circuits/componentsmay be described as performing a task or tasks, for convenience in thedescription. Such descriptions should be interpreted as including thephrase “configured to.” Reciting a unit/circuit/component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. §112, paragraph six interpretation for thatunit/circuit/component. More generally, the recitation of any element isexpressly intended not to invoke 35 U.S.C. §112, paragraph sixinterpretation for that element unless the language “means for” or “stepfor” is specifically recited.

DETAILED DESCRIPTION OF EMBODIMENTS

During the manufacture of a semiconductor memory circuit, differences inlithography, implant levels, etc., may result in differences inelectrical characteristics between data storage cells that are otherwiseintended to be identical in characteristics and performance. In somecases, the variation of the electrical characteristics of a data storagecell may be sufficiently large that the data storage cell may notfunction (e.g., read or write) under normal operating conditions of thememory circuit, resulting in the data storage cell being identified as afailure and requiring replacement with a redundant data storage cell.Adding redundant data storage cells to the memory circuit to compensatefor data storage cells with non-ideal electrical characteristics mayresult in additional chip area and power consumptions. The embodimentsillustrated below may provide techniques to identify and compensate fordata storage cells with non-ideal electrical characteristics.

FIG. 1 illustrates a data storage cell according to one of severalpossible embodiments. In the illustrated embodiment, data storage cell100 includes a true I/O 102 denoted as “bt,” a complement I/O 103denoted as “bc,” and a selection input 101 denoted as “wl.”

In the illustrated embodiment, bt 102 is coupled to selection transistor104 and bc 101 is coupled to selection transistor 105. Selectiontransistor 104 and selection transistor 105 are controlled by wl 101.Selection transistor 104 is further coupled to pull-up transistor 108and pull-down transistor 106 through node 110, and selection transistor105 is further coupled to pull-up transistor 109 and pull-downtransistor 107 through node 111. Pull-up transistor 108 and pull-downtransistor 106 are controlled by node 111, and pull-up transistor 109and pull-down transistor 107 are controlled by node 110.

It is noted that although selection transistors, pull-up transistors,pull-down transistors, and pre-charge transistors may be illustrated asindividual transistors, in other embodiments, any of these transistorsmay be implemented using multiple transistors or other suitablecircuits. That is, in various embodiments, a “transistor” may correspondto an individual transistor or other switching element of any suitabletype (e.g., a field-effect transistor (FET)), or to a collection oftransistors.

At the start of the storage operation true I/O 102 and complement I/O103 may both be high and selection input 101 may be low. It is notedthat in this embodiment, low refers to a voltage at or near groundpotential and high refers to a voltage sufficiently large to turn onn-channel metal oxide semiconductor field-effect transistors (MOSFETs)and turn off p-channel MOSFETs. In other embodiments, other circuitconfigurations may be used and the voltages that constitute low and highmay be different. During the storage, or write, operation, selectioninput 101 may be switched high which couples true I/O 102 to node 110and complement I/O 103 to node 111. To store a logical 1 into datastorage cell 100, complement I/O 103 may be switched to a low. Sinceselection transistor 105 is on, node 111 is also switched low. The lowon node 111 activates pull-up transistor 108 which charges node 110high. The high on node 110, in turn, activates pull-down transistor 107,which further reinforces the low on node 111 establishing regenerativefeedback. Once this regenerative feedback between nodes 110 and 111 hasbeen established, selection input 101 may be switched low turning offselection transistor 104 and selection transistor 105, isolating node110 from true I/O 102 and node 111 from complement I/O 103. The methodof storing a logical 0 may be similar. Selection input 101 may beswitched high and true I/O 102 may be switched low. Selection transistor104 couples the low on true I/O 102 to node 110, which activates pull-uptransistor 109. The high on node 111 activates pull-down transistor 106,reinforcing the low on node 110 and establishing the regenerativefeedback. Data storage cells that store data via regenerative feedbackare commonly referred to as static cells.

In the illustrated embodiment, data storage cell 100 outputs its storeddata as the difference in voltage between true I/O 102 and complementI/O 103. (Data stored as the difference between two voltages may also bereferred to herein as “differentially encoded”.) At the start of theoutput process, true I/O 102 and complement I/O 103 may both be high andselection input 101 may be low. Asserting selection input 101 activatesselection transistor 104 and selection transistor 105. If node 111 islow and node 110 is high, then a current will flow through selectiontransistor 105 and pull-down transistor 107 causing a reduction involtage on complement I/O 103. If node 110 is low and node 111 is high,then a current will flow through selection transistor 104 and pull-downtransistor 106 causing a reduction in voltage on true I/O 102. Foreither data state, the current that the data storage cell sinks fromeither the true I/O 102 or complement I/O 103 is referred to as the readcurrent of the cell.

Ideally, the electrical characteristics of pull-down transistor 106 andpull-down transistor 107 would be identical, as would be the electricalcharacteristics of selection transistor 104 and selection transistor105. Furthermore, in an ideal circuit, it might be desirable thatpull-down transistor 106 and pull-down transistor 107 in one datastorage cell in a memory device have identical electricalcharacteristics to pull-down transistor 106 and pull-down transistor 107in another data storage cell in the memory device. However, during thesemiconductor manufacturing process, differences in lithography,fluctuations in dopant levels, etc., may result in these transistorshaving different electrical characteristics (e.g., saturation current).Aging effects induced by, e.g., hot-carrier injection may also change atransistor's electrical characteristics over time. Variation, due toboth manufacturing and aging effects, in pull-down transistor 106,pull-down transistor 107, selection transistor 104 and selectiontransistor 105 from one data storage cell to another may result invariation in read currents, and, therefore variation in output voltagesfor the same stored data.

In some cases, the variation in the electrical characteristics of thetransistors may result in larger than average output voltages when thestorage cell is read. Data storage cells that generate larger thanaverage output voltages may be referred to as strong cells. In somecases, the variation in the electrical characteristic of the transistorsmay result in smaller than average output voltages when the storage cellis read. Data storage cells that generate smaller than average outputvoltages may be referred to as weak cells. If the value of the outputvoltage generated by a weak storage cell is sufficiently small, it maynot be possible to properly determine the data stored in the datastorage cell in a timely manner, or not at all, because the outputvoltage may not be able to overcome imbalances and signal noise within asense amplifier.

It is noted that the number of transistors and the connectivity shown inFIG. 1 are merely an illustrative example, and that in otherembodiments, other numbers, types of transistors, and/or circuitconfigurations may be employed. It is also noted that in other datastorage cell embodiments, other storage mechanisms may be employed. Forexample, a capacitor (as, e.g., in a dynamic random access memory(DRAM)), transistor implants (as, e.g., in a depletion programmableread-only memory (ROM)), or a floating gate structure (as in asingle-bit or multi-bit non-volatile or flash memory) may be used tostore data in a data storage cell.

FIG. 2 illustrates possible waveforms resulting from the operation ofthe embodiment of the data storage cell shown in FIG. 1. At time t₀ 205,the selection input 101 is asserted (waveform 201). Depending on thevalue of the stored data, either true I/O 102 or complement I/O 103 willbegin to discharge (waveform 203). At time t₁ 206, the small signaldifferential between true I/O 102 and complement I/O 103 is amplified bya sense amplifier. The system including one or more data storage cellsmay be modeled as a capacitor and current source. The capacitorrepresents the total capacitance present on either true I/O 102 orcomplement I/O 103 which may include the junction capacitance of otherdata storage cells I/O ports and the capacitance of the interconnectbetween the data storage cells. The current source is the read currentof the data storage cell. With this model, the voltage on the low-goingI/O from time t₀ to time t₁ can be estimated using equation 1.

$\begin{matrix}{{v(t)} = {\frac{1}{C}{\int_{t_{0}}^{t_{1}}{{i(t)}\ {\mathbb{d}t}}}}} & (1)\end{matrix}$

Over a limited range of time and voltages, the read current can betreated as a constant. This allows the equation to be simplified asshown in equation 2. For a constant load capacitance, the voltage changeon the low-going I/O is proportional to the read current of the datastorage cell. If the read current of the data storage cell is less thanaverage, then the change in voltage on the low-going I/O will be less(waveform 204), resulting in a smaller differential voltage at the timethe sense amplifier is activated. If the read current of the datastorage cell is larger than average, then the change in voltage on thelow-going I/O will be greater (waveform 202), resulting in a largerdifferential at the time the sense amplifier is activated. It is notedthat the waveforms shown in FIG. 2 are merely an illustrative exampleand that, in other embodiments, differing waveform behavior may bepossible.

$\begin{matrix}{{\Delta\; v} = \frac{i_{read}}{C\left( {t_{1} - t_{0}} \right)}} & (2)\end{matrix}$

FIG. 3 illustrates an embodiment of a memory sub-array which includes adata output 311 denoted as “dout,” a pre-charge control input 308denoted as “pchgb,” a sense amplifier enable input 309 denoted as“saen”, a true data selection input 314 denoted as “dselt,” a complementdata selection input 315 denoted as “dselc,” and a bias input 313. Theillustrated embodiment also includes one or more column selection inputs307 denoted as “cs” and one or more row selection inputs 304 denoted as“rs.”

In the illustrated embodiment, columns 301 a, 301 b, 301 c, and 301 dare coupled to the inputs of column multiplexer 302 through bit lines312. The differentially encoded output of column multiplexer 302 iscoupled to the differential inputs of sense amplifier 303 through nodes310 a and 310 b, and the output of sense amplifier 303 is coupled todout 311. Current injector 304 is also coupled to the differentialinputs of sense amplifier 303.

Each column 301 may include one or more of data storage cell 100. Forexample, the individual bit lines bt 102 of each data storage cell 100within in a column 301 may be coupled together to form a true bit line312 of column 301. Likewise, the individual bit lines be 103 of eachdata storage cell 100 within column 301 may be coupled together to forma complement bit line 312 of column 301. Individual word lines wl 101 ofeach data storage cell 100 within column 301 may coupled to a respectiveone of row select signals rs 306 such that when a given rs 306 isasserted, the corresponding data storage cell 100 creates adifferentially encoded output on the true bit line and complement bitline of column 301, while the bit line outputs of the remaining datastorage cells 100 within column 301 remain quiescent. In otherembodiments, the data storage cells may be dynamic storage cells,single-bit or multi-bit non-volatile storage cells, or mask programmableread-only storage cells. It is noted that in some embodiments, the datastorage cell may transmit data in a single-ended fashion. In such cases,only a single bit line per column is required.

In some embodiments, column multiplexer 302 may contain one or more passgates controllable by cs 307. The input of each pass gate may be coupledto the either the true or complement bit line output from one of columns301 a, 301 b, 301 c, and 301 d. The output of each pass gate coupled toa true bit line is coupled to the true output of column multiplexer 302in a wired-OR fashion, and the output of each pass gate coupled to acomplement bit line is coupled to the complement output of columnmultiplexer 302 in a wired-OR fashion. In other embodiments, columnmultiplexer 302 may contain one or more logic gates configured toperform the multiplexer selection function.

Sense amplifier 303 may use analog amplification techniques in someembodiments. In other embodiments, sense amplifier 303 may employ alatch based amplification technique. Current injector 304 may containbias transistors and selection transistors as will be described inreference to FIG. 5 and FIG. 6.

In some embodiments, the illustrated sub-array 300 may operate asfollows. Referring collectively to FIG. 3 and the flowchart illustratedin FIG. 4, the operation may start by initializing the sub-array (block401) by setting pchgb 308 low and setting rs 306, cs 307, and saen 309to inactive states. Once sub-array 300 has been initialized, one of rs306 may be asserted (block 402) selecting a data storage cell in each ofcolumns 301 a, 301 b, 301 c, and 301 d. One of cs 307 may then beasserted (block 403), causing column multiplexer 302 to output dataselected from one of bit lines 312.

The operation then depends on whether or not sub-array 300 is operatingin test mode (block 404). When sub-array 300 is not operating in testmode, pchgb 308 may be set high (disabling pre-charge) and saen 309 maybe asserted causing sense amplifier to amplify the difference betweennodes 310 a and 310 b and couple the amplified result to dout 311 (block407). Sub-array 300 may then be re-initialized by de-asserting saen 309,and the asserted one of rs 306 and cs 307, and setting pchgb 308 low(block 401).

When sub-array 300 is operating in test mode, the operation then dependson the value of the test data previously loaded into the selected datastorage cell (block 404). When a logical 1 was loaded into the selecteddata storage cell, dselc 315 may be set low causing current injector 304to source current onto the complement input of sense amplifier 303. Withthe additional current, the equation governing the change of voltage onthe complement input of sense amplifier 303 can re-written as shown inEquation 3. Since the change in voltage is proportional to the totalcurrent, the change in voltage on the complement input of senseamplifier 303 may be reduced. Once the additional current is beingsource to the complement input of sense amplifier 303, the amplificationoperation (block 407) and initialization operation (block 401) canproceed as described above.

$\begin{matrix}{{\Delta\; v} = \frac{\left( {i_{read} - i_{injected}} \right)}{C\left( {t_{2} - t_{0}} \right)}} & (3)\end{matrix}$

FIG. 5 illustrates an embodiment of a current injector for use withdifferentially encoded data. The illustrated embodiment includes a truedata port 501 and a complement data port 502, respectively denoted as“datat” and “datac,” as well as a bias input 511. The embodiment furtherincludes a true data selection input 503 and a complement data selectioninput 504, respectively denoted as “dselt” and “dselc.”

In the illustrated embodiment, datat 501 is coupled to selectiontransistor 505 and datac 502 is coupled to selection transistor 506.Selection transistor 505 is controlled by dselt 503 and selectiontransistor 506 is controlled by dselc 504. Selection transistor 505 isfurther coupled to bias transistor 507 and selection transistor 506 isfurther coupled to bias transistor 510. Bias transistor 509 and biastransistor 510 are controlled by bias 511. In some embodiments, thetransconductance of bias transistor 509 is the same as thetransconductance of bias transistor 510. Bias transistor 509 and biastransistor 510 may be “matched”, that is, the physical design of the twotransistors follows additional design rules to minimize variation inelectrical characteristics between the transistors resulting fromdifferences in lithography, variations in dopant levels, etc. In otherembodiments, current injector 500 may include self-biasing circuitry togenerate bias 511 internal to current injector 500.

During normal read operation, bias 511 may be set high, turning off biastransistors 509 and 510. Dselt 503 and dselc 504 may both be set high,de-activating selection transistors 505 and 506, and de-coupling datat501 and datac 502 from their respective bias transistors. During testread operation, bias 511 may be set to an analog voltage level causing acurrent to flow through bias transistors 509 and 510. When test data isto be read from a data storage cell, either dselt 503 or dselc 504 maybe set low depending on the anticipated value of the test data. Forexample, it the test data to be read is a logical 1, then dselc 504 maybe set low activating selection transistor 506 and sourcing current frombias transistor 510 to datac 502.

FIG. 6 illustrates a variant of current injector 500 that providesmultiple current levels. In the illustrated embodiment, current injector600 includes a number of input and I/O ports similar to current injector500: a true data I/O 601, a complement data I/O 602, respectivelydenoted as “datat” and “datac,” a true data selection input 603 and acomplement data selection input 604, respectively denoted as “dselt” and“dselc,” and a bias input 611. In contrast to current injector 500,current injector 600 includes a first current level selection input 608denoted as “lsel1,” and a second current level selection input 607denoted as “lsel2.”

As shown in FIG. 6, datat 601 is coupled to selection transistor 617 anddatac 602 is coupled to selection transistor 618. Selection transistor617 is controlled by dselt 603 and selection transistor 618 iscontrolled by dselc 604. Selection transistor 617 is further coupled toselection transistor 609 and selection transistor 611. Selectiontransistor 618 is further coupled to selection transistor 612 andselection transistor 610. Selection transistor 609 and selectiontransistor 610 are controlled by lsel1 608, and selection transistor 611and selection transistor 612 are controlled by lsel2 607. Selectiontransistor 609 is coupled to bias transistor 613, and selectiontransistor 611 is coupled to bias transistor 615. Selection transistor612 is coupled to bias transistor 616, and selection transistor 610 iscoupled to bias transistor 614. Bias transistors 613, 614, 615, and 616are controlled by bias 611.

In some embodiments, the transconductance of bias transistors 613, 614,615, and 616 may be equal. In other embodiments, the transconductance ofbias transistors 613 and 614 may be equal, and the transconductance ofbias transistors 615 and 616 may be equal but different from thetransconductance value of bias transistors 613 and 614, allowing fordifferent current levels. The differing transconductance values may beimplement by changing the electrical characteristics of the transistors(e.g., adjusting dopant levels), or by adjusting the physical size ofthe transistor. For example, making bias transistor 615 twice the sizeof bias transistor 613 may allow bias transistor 615 to source twice asmuch current. In other embodiments, the bias transistors may becontrolled by multiple bias signals. It is noted that in otherembodiments, different configurations, types, and numbers of transistorsmay be employed.

During normal read operation, bias 611 may be set high to turn off biastransistors 613, 614, 615, and 616. Dselt 603 and dselc 604 are both sethigh, turning off selection transistors 617 and 618, isolating currentinjector 600 from its load. During test operation, bias signal 611 maybe set to analog voltage level which causes current to flow in biastransistors 613, 614, 615, and 616. In some embodiments, bias signal 611may be generated by voltage reference circuit designed to supply aconstant voltage over a range of supply voltages and temperatures. Thebias signal may be generated as part of a current mirror circuit andbias transistors 613, 614, 615, and 616 may comprise the last stages ofthe mirror. In other embodiments, bias signal 611 may be suppliedexternally by a tester or other suitable hardware. When test data is tobe read from a data storage cell, either dselt 603 or dselc 604 may beset low depending on the anticipated value of the test data, and eitherlsel1 608 or lsel2 607 may be set low depending on the desired currentlevel. For example, it the test data to be read is a logical 1 andcurrent level 2 is to be used, then dselc 604 and lsel2 may be set lowactivating selection transistors 618 and 612, allowing the currentprovided by bias transistor 616 to flow to datac 602.

FIG. 7 illustrates a memory according to one of several possibleembodiments. In the illustrated embodiment, memory 700 includes data I/Oports 709 denoted “dio,” an address bus input 712 denoted “add,” modeselection inputs 711 denoted “mode,” and a clock input 710 denoted“clk.”

In the illustrated embodiment, memory 700 includes sub-arrays 701 a, 701b, and 701 c, timing and control unit 702, address decoder 703, andaddress comparator 704. Sub-arrays 701 a, 701 b, and 701 c mayincorporate some or all of the features described above with respect tosub-arrays 300. Timing and control unit 702 is coupled to provide adecoder enable signal 708 to address decoder 703 and address comparator704, and control signals 705 to sub-arrays 701 a, 701 b, and 701 c. Insome embodiments, control signals 705 may include a pre-charge signal, asense amplifier enable signal, a true data selection signal, acomplement data selection, a first load selection signal, and a secondload selection signal that may operate as described above with respectto sub-array 300 and load circuits 500 and 600. In other embodiments,timing and control unit 702 may include a test unit 716 that may performbuilt-in self-test (BIST) functions.

In some embodiments, timing and control unit 702 may be configured toprovide bias signal 714. Timing and control unit 702 may include one ormore current mirrors and temperature and supply independent voltageand/or current reference circuits (e.g., a band gap reference). In otherembodiments, bias signal 714 may be supplied externally to memory 700 bya tester or other circuit blocks in a system-on-a-chip (SOC)implementation.

Address decoder 703 is coupled to provide row selects 706 and columnselects 707 to sub-arrays 701 a, 701 b, and 701 c, in response to theassertion of decoder enable signal 705 and the address value on addressbus 712. Address comparator 704 is coupled to provide read-missindication signal 708 to timing and control unit 702 based upon acomparison of the address value on add 712 to a collection of addressvalues previously determined to select weak data storage cells insub-arrays 701 a, 701 b, and 701 c. In some embodiments, addresscomparator 704 may include a storage unit 715 configured to storeaddress values that select weak data storage cells.

FIG. 8. Illustrates a possible method of operating memory 700 to testfor weak data storage cells. Referring collectively to FIG. 7 and theflowchart illustrated in FIG. 8, the operation starts in block 801. Thevalue presented to add 712 is set to zero (block 802). The operationthen depends on the value presented to add 712. When the value presentedto add 712 exceeds the maximum address of memory 700, the test ends(block 807). When the value presented to add 712 is less than themaximum address of memory 700, mode 711 may be set for a writeoperation, test data may be presented to dio 709, and clk 710 may beasserted, writing the test data into the data storage cells selected bythe value presented to add 712 (block 804).

Once the test data has been loaded, memory 700 may be re-initialized.Mode 711 may be set for read and test operation and clk 710 may beasserted initiating the read and comparison operation as will bedescribed in reference to FIG. 9 (block 805). When the read andcomparison operation has completed, memory 700 may be re-initialized andthe value presented to add 712 may be incremented (block 806) and thevalue checked against the maximum address for memory 700 (block 803). Itis noted that operations shown in FIG. 8 are merely an illustrativeexample and that in actual circuit operation, other operations and orderof operations may be possible.

A possible method of operating memory 700 to read and compare previouslyloaded test data is illustrated in FIG. 9. Referring collectively toFIG. 7 and the flow chart illustrated in FIG. 9, the operation may beginby de-asserting clk 710 to initialize memory 700 (block 901). Mode 711may be set for normal read operation and clk 710 may be asserted whichcauses timing and control unit 702 to assert decoder enable signal 708.Address decoder 703 decodes the address presented to add 712 (block 903)in response to the assertion of decoder enable signal 708, and mayassert one of row selects 706, and one of column selects 707 (block903), selecting a data storage cell in each of sub-arrays 701 a, 701 b,and 701 c. Timing and control unit 702 may then assert the appropriatesignal in control signals 705 to activate the sense amplifiers insub-arrays 701 a, 701 b, and 701 c, causing them to amplify the datafrom the selected data storage cells and output the amplified data todio 709 (block 904).

The operation then depends on value of data output on dio 709 (block905). When the data output on dio 709 does not match the originallyloaded test data, the selected data storage cells may contain one ormore hard failures (block 906). In this test flow, no further action istaken and the test of data storage cells at the given address iscomplete (block 915). When the data output on dio 709 matches theoriginally loaded test data, further testing may be necessary and clk710 may be de-asserted, re-initializing memory 700 (block 907). Mode 711may be set for test read operation and clk 710 may be asserted. Inresponse to the assertion of clk 710, timing and control unit 702 mayassert decoder enable 708, causing decoder 703 to decode the addresspresented to add 712 (block 908). Address decoder 703 may then assertone of row selects 706 and one of column selects 707, selecting a datastorage cell in each of the sub-arrays 701 a, 701 b, and 701 c (block909). In some embodiments, timing and control unit 702 may then assertthe necessary control signals 705 and bias signal 714 to activatecurrent injectors in sub-arrays 701 a, 701 b, and 701 c (block 910).Dependent upon original test data, the current injectors source currentto either the true input or the complement input of sense amplifiers.Timing and control unit 702 may then assert the necessary controlsignals 705 to activate the sense amplifiers, causing the senseamplifiers to amplify the data from the selected data storage cells andcouple the amplified data to dio 709.

The newly-read value of the data output on dio 709 may be comparedagainst the originally loaded test data (block 912). When the dataoutput on dio 709 matches the originally loaded test data, the selecteddata storage cells have sufficient read current to overcome theadditional load provided by the load circuits. The address that selectedthese data storage cells may be noted as containing cells of normalstrength (block 914). The test operation at the given address may becomplete (block 915). When the data output on dio 709 does not match theoriginally loaded test data, one or more of the selected data storagecells do not have sufficient read current to overcome the additionalload provided by the load circuits. The address that selected these datastorage cells may be noted as containing weak data storage cells (block913). The test operation at the given address may then be complete(block 915). In some embodiments, the address may be loaded into storageunit 715 such that when the given address is encountered in a subsequentread access to memory 700, address comparator 704 asserts misreadindication signal 713. It is noted that during actual circuit operation,some or all of the operations illustrated in FIG. 9 may occur in adifferent order, or may occur concurrently rather than sequentially.

Turning now to FIG. 10, a block diagram of a system is illustrated. Inthe illustrated embodiment, the system 1000 includes an instance of arandom access memory (RAM) 1002 and a read-only memory (ROM) 1003 eachof which each may include one or more sub-arrays that may incorporatesome or all of the features described above with respect to sub-array300.

The illustrated embodiment also includes a CPU 1001 which may includeone or more local storage units 1009. For example, CPU 1001 may includea Cache Data RAM, a Tag RAM, one or more register files, and one or moreFIFOs. Each one of the local storage units 1009 may include one or morecurrent injectors that may incorporate some or all of the featuresdescribed above with respect to current injectors 500 and 600. In someembodiments, CPU 1001 may include a test unit 1010 configured to operatethe current injectors. Test unit 1010 may include one or more currentmirror and supply and temperature independent voltage and/or currentreferences to generate the necessary bias signal. Additionally, theillustrated embodiment includes an I/O adapter 1005, a display adapter1004, a user interface adapter 1006, and a communication adapter 1007.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. An apparatus, comprising: a plurality of data storage cells; a sense amplifier configured to amplify data stored in a selected one of the plurality of data storage cells; and a current injector unit coupled to the sense amplifier, wherein the current injector unit is configured to source a first current to an input of the sense amplifier dependent upon a control signal.
 2. The apparatus of claim 1, wherein to source the first current to the input of the sense amplifier, the current injector unit is configured to source the current to the input of the sense amplifier dependent upon a bias signal.
 3. The apparatus of claim 1, wherein the current injector unit is further configured to sink a second current from the input of the sense amplifier.
 4. The apparatus of claim 1, wherein an assertion of the control signal is responsive to activation of a test mode.
 5. The apparatus of claim 1, wherein an assertion of the control signal is responsive to a determination that the selected one of the plurality of data storages cells comprises a weak data storage cell.
 6. The apparatus of claim 1, further comprising a comparator unit configured to: compare an address corresponding to a selected one of the plurality of data storage cells to one or more previously identified addresses corresponding to weak data storage cells; and assert the control signal responsive to a determination that the address corresponding to the selected one of the plurality of data storage cells matches one of the one or more previously identified addresses corresponding to weak data storage cells.
 7. A method, comprising: receiving, by a memory, a first address from which to read stored data, wherein the first address corresponds to a first data storage cells of a plurality of data storage cells; comparing the first address to one or more previously identified addresses, wherein each of the one or more previously identified address corresponds to a respective group of one or more data storage cells, and wherein each group of one or more data storage cells includes at least one weak data storage cell; sourcing a first current to an input of a first sense amplifier coupled to the first data storage cell responsive to a determination that the first address matches one of the one or more previously identified addresses; and amplifying data stored in the first data storage cell using the sense amplifier.
 8. The method of claim 7, wherein comparing the first address to the one or more previously identified addresses comprises asserting a misread indication signal responsive to a determination that the first address matches one of the one or more previously identified addresses.
 9. The method of claim 7, wherein the first sense amplifier is coupled to a current injector unit, and further comprising providing a bias signal to the current injector unit.
 10. The method of claim 7, further comprising testing the plurality of data storage cells.
 11. The method of claim 10, wherein testing the plurality of data storage cells comprises: storing test data into a given data storage cell of the plurality of data storage cells; source a second current to an input of a second sense amplifier coupled to the given data storage cell of the plurality of data storage cells; and amplifying data stored in the given data storage cell using the second sense amplifier.
 12. The method of claim 11, further comprising: comparing the test data and the amplified data; and determining a strength of the given data storage cell dependent upon the comparison.
 13. The method of claim 7, further comprising: receiving, by the memory, a second address from which to read stored data, wherein the second address corresponds to a second data storage cells of a plurality of data storage cells; comparing the second address to the one or more previously identified addresses; and sourcing a second current to an input of a second sense amplifier coupled to the second data storage cell responsive to a determination that the second address matches one of the one or more previously identified addresses.
 14. The method of claim 13, wherein the second current is larger than the first current.
 15. A system, comprising: a processing unit; and a memory coupled to the processing unit, wherein the memory is configured to: receive an address from the processing unit corresponding to at least one of a plurality of data storage cells; compare the address to one or more previously identified addresses corresponding to weak data storage cells; source a current to an input of a sense amplifier coupled to the at least one data storage cell responsive to a determination that the address matches one of the one or more previously identified addresses; and amplify data storage in the at least one data storage cell using the sense amplifier.
 16. The system of claim 15, wherein to compare the address to the one or more previously identified addresses, the memory is further configured to assert a misread indication signal responsive to the determination that the address matches one of the one or more previously identified addresses.
 17. The system of claim 15, wherein to source the current to the input of sense amplifier, the memory is further configured to provide a bias signal to a current injector unit coupled to the sense amplifier.
 18. The system of claim 15, wherein the memory is further configured to: test the plurality of data storage cells to generate test results; and determine the one or more previously identified addresses dependent upon the test results.
 19. The system of claim 18, wherein to test the plurality of data storage cells, the memory is further configured to: store test data into a given data storage cell of the plurality of data storage cells; source a second current to a second sense amplifier coupled to the given data storage cell; amplify data stored in the given data storage cell using the second sense amplifier.
 20. The system of claim 19, wherein the memory is further configured to: compare the test data and the amplified data; and determine a strength of the given data storage cell dependent upon the comparison. 